1. Field of the Invention
The present invention relates to a liquid phase growth method an a liquid phase growth apparatus and, more particularly, to a liquid phase growth method or a liquid phase growth apparatus that can be applied to production of such devices as solar cells or photosensors.
2. Related Background Art
Emission of greenhouse-effect gases such as carbon dioxide and nitrogen oxides, resulting from combustion of petroleum in thermal power generation, combustion of gasoline in engines of cars, and so on, is responsible for deterioration of the global environments. There are also worries that crude oil will have been exhausted in the future, and attention thus has been focused on power generation using the solar cells.
Since thin film crystalline Si solar cells have a thin electricity-generating layer and use a small amount the raw material of Si, they can be produced at lower cost. Since the electricity-generating layer is crystalline Si, higher conversion efficiency and less deterioration can be expected, as compared with the solar cells amorphous Si or the like. Further, the thin film crystalline Si solar cells can be bent to some extent, and they can thus be used as being bonded to a curved surface, e.g. a body of a car, a household electrical appliance, a rood file, and so on.
For implementing the thin film crystalline Si solar cells, Japanese Patent Application Laid-Open No. 8-213645 discloses that thin-film single-crystal Si epitaxially grown is separated through a porous Si layer. FIG. 20 is a sectional view to show a method for forming a solar cell of thin film Si described in Japanese Patent Application Laid-Open No. 8-213645. In the figure, reference numeral 101 designates a Si wafer, 102 a porous Si layer, 103 a p.sup.+ Si layer, 104 a p.sup.- Si layer, 105 a n.sup.+ Si layer, 106 a protective film, 109, 111 an adhesive, and 110, 112 a jig. In the production method of solar cell of FIG. 20, the porous Si layer 102 is formed in the surface of Si wafer 101 by anodization. After that, the p.sup.- Si layer 103 is epitaxially grown on the porous Si layer 102 and then the p.sup.- Si layer 104 and n.sup.- Si layer 105 are further grown thereon. Then the protective layer 106 is formed. After that, the adhesive 111, 109 is applied onto the protective layer 106 and onto the Si wafer 101, which are bonded to the jig 112, 110. After that, tensile force is exerted on the jig 112, 110 so as to separate the Si wafer 101 from the epitaxial Si layers 103, 104, 105 through the porous Si layer 102. Then the solar cell is formed in the epitaxial Si layers 103, 104, 105 and the Si wafer 101 is again used in like steps, thereby achieving cost reduction.
There are liquid phase growth methods as methods for forming single-crystal Si or polycrystal Si. The liquid phase growth methods permit the thick Si layers necessary for the electricity-generating layer of a solar cell to be produced at lower cost than such methods as CVD (Chemical Vapor Deposition). A specific example of the liquid phase growth method is disclosed in U.S. Pat. No. 4,778,478. FIG. 21 is a sectional view of a liquid phase growth apparatus of a sliding method disclosed in U.S. Pat. No. 4,778,478. In the figure, reference numeral 50 denotes a sliding boat of a fire-resistive material such as graphite, 54, 56 liquid baths, 58 a movable slide comprised of a metal substrate, 60 a recessed part in the bottom surface of the boat, 63 a barrier layer, 68, 70 solvents, 72 a section for adhering a transparent conductive electrode, 75 a nozzle for forming an antireflection film, 74 a chamber thereof, 76 a wheel, and 78 a nozzle for forming the barrier layer. First, the movable slide 58 rolled up around the wheel 76 is unrolled and the barrier layer 63 is formed thereon by the nozzle 78. Then semiconductor layers to become the electricity-generating layer are formed by liquid phase growth from the solvents 68, 70 in the baths 54, 56. Thereafter, the transparent electrode is formed at the section 72 for adhering the transparent conductive electrode, and the antireflection film is formed by the nozzle 75, thereby completing the solar cell. This liquid phase growth method of the sliding method has high efficiency of liquid phase growth and is thus advantageous in mass production of solar cells.
U.S. Pat. No. 5,544,616 discloses another liquid phase growth apparatus of a dipping system. A sectional view of this liquid phase growth apparatus is illustrated in FIG. 22. In the figure, numeral 201 represents an exit, 202 a quartz crucible, 203 a graphite boat, 204 a heater, 205 an inlet of argon gas, 206 a thermocouple, 208 a lid, 209 an insulating region, and 210 a support base of graphite. The apparatus of U.S. Pat. No. 5,544,616 forms a semiconductor layer on a growth substrate by dipping the growth substrate in the solvent stored in the quartz crucible 202.
In the case wherein the semiconductor layer is intended to be formed by liquid phase growth on a wafer as a substrate as it is, the sliding boat larger than the size of the wafer has to be prepared in the sliding method, e.g., in U.S. Pat. No. 4,778,478. It is, however, not easy to fabricate the sliding boat in a large scale, because it is made of the heat-resistant material such as graphite. In this aspect, the liquid phase growth apparatus of the sliding method is disadvantageous in producing large-area devices such as the solar cells or the photosensors. Therefore, the larger the size of the wafer, the more disadvantageous the use of the liquid phase growth apparatus of the sliding method.
Further, since the liquid phase growth apparatus using the dipping system as disclosed in U.S. Pat. No. 5,544,616 etc. excels in the liquid phase growth of large areas, it is advantageous in the case of the wafer being used as a substrate as it is, but is disadvantageous in continuous formation of the semiconductor layers such as the p-layer and n-layer.